1. Field of the Invention
The present invention relates to an ion mobility spectrometer.
2. Description of the Related Art
There is a demand for a small-sized and portable monitoring device for speedily and accurately detecting harmful matters such as explosives, poison gas, environmental pollutants. The monitoring device based on mass spectrometry is excellent in sensitivity and selectivity and is capable of performing accurate measurement with little false detection. However, the mass spectrometer needs high vacuum, and therefore, is large in size. On the other hand, the ion mobility spectrometer is capable of detecting ions by spatially separating ions under atmospheric pressure based on the difference in the ion mobility. The ion mobility spectrometer has a feature in which a vacuum system is not needed and therefore the device is easily miniaturized. However, the ion mobility spectrometry is inferior to the mass spectrometry in selectivity and has a problem in that false detection often occurs.
Japanese Patent Publication No. 2006-107929 discloses a tandem ion mobility spectrometer. The similar device is disclosed also in Stormy L. Koeniger, Samuel I. Merenbloom, Stephen J. Valentine, Martin F. Jarrold, Harold R. Udseth, Richard D. Smith, and David E. Clemmer, Anal. Chem. 2006;78:4161. In these tandem ion mobility spectrometers, first and second separation sections (so-called drift tubes) for spatially separating ions based on the difference in the ion mobility are arranged in series, and an ion dissociation section for dissociating ions is provided at an intermediate section therebetween. Among the ions spatially separated by the first drift tube, ions having predetermined ion mobility are introduced to the ion dissociation section and dissociated by some means in the ion dissociation section. Fragment ions generated by the ion dissociation section are spatially separated according to difference in ion mobility while passing through the second drift tube, and reach a detector at different times. Even when two kinds of substances having approximately the same ion mobility exist, these tandem ion mobility spectrometers are capable of distinguishing these kinds of substances as long as their fragment ions are different in ion mobility to such an extent they can be separated.
Japanese Patent Translation Publication No. 2005-524196 and Japanese Patent Publication No. 2003-329646 disclose a technique in which various types of ions are spatially separated and captured according to the difference in ion mobility in an interior of the drift tube of the ion mobility spectrometer. The principle used is based on the following point. Namely, when gas flow, which opposes in an ion traveling direction and has linear velocity gradually decreasing, is generated in the interior of the drift tube, movement of ion stops at a position where ion mobility and resistance force due to gas flow are balanced with each other. After ions are captured and accumulated by this technique, drift voltage is increased and the ions are discharged from the drift tube to perform detection. Using such technique can improve sensitivity and resolution of the ion mobility spectrometer, resulting in improved selectivity.